| Compared with the traditional AC distribution power system(DPS),the DC DPS has the advantages of excellent power quality,large power supply capacity,high efficiency,high reliability,easy access to renewable energy sources,and flexible voltage control.Therefore,in order to promote photovoltaic consumption,distributed photovoltaic accessed to the DC DPS from multiple terminals has become an active exploration.Recently,DC distribution power technology has developed rapidly and DC DPS has emerged the characteristics of several voltage levels,multiple DC buses,and many converters,such as medium and low voltage DC DPS.The increasing complexity of the system topology brings the following two main problems: one is the DC bus voltage instability problem caused by impedance mismatch is more prominent;the other is how to achieve coordinated control of the converters to ensure stable system operation.To this end,this article will study the stability analysis and coordinated control strategies of medium and low voltage DC DPS.The main patrs of this dissertation are as follows:1.The first part summarizes current main impedance criteria used for stability analysis of DC DPS,which can be divided into two categories: impedance ratio criterion and passivity-based impedance criterion.Since admittance is the inverse of impedance and can be converted into each other on the circuit,this paper proposes a stability criterion for DC DPS based on global admittance to analyze the system stability from the perspective of admittance.Then,the proposed admittance criterion is compared with the existing impedance criteria to shows their main characteristics,as well as advantages and disadvantages.2.The current impedance criteria do not consider the case where the DC DPS has multiple voltage levels.Cosequently,the second part proposes a general impedance criterion for the stability analysis of medium and low voltage DC DPS.First of all,a unified form of the system is obtained regardless of its structures and operating modes,based on classification of any converter as either a bus voltage controlled converter(BVCC)or a bus current controlled converter(BCCC).It should be noted that the DC transformer connected to the medium voltage DC bus and low voltage DC bus could adopt output voltage or output current control modes.After that,the two-port small signal model and general impedance criterion of the system is derived.Finally,a simulation model and an experimental prototype are established to validate the effectiveness of the proposed criterion.3.In the third part,a decentralized unified control strategy for the medium and low voltage DC DPS is proposed.The photovoltaic power generation unit,energy storage unit and fuel cell unit on the DC bus adopt decentralized control strategies to adjust their working modes according to the DC bus voltage signal.The DC transformer utilizes a unified control strategy which integrates the functions of power control,medium-voltage and low-voltage DC bus voltage regulation.The DC transformer can automatically adjust the magnitude and direction of the transmission power based on normalized values of medium voltage and low voltage DC bus voltage deviations,and realize the global power sharing of the system.Finally,the simulation model of medium and low voltage DC power distribution system is built in MATLAB/Simulink,and simulation analysis of various operating states and scenarios is carried out to verify the effectiveness of the proposed control strategy. |
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